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张业猛, 沈迎芳, 王英芳, 姚品雅, 王海庆
蒺藜苜蓿MtLEA5B的克隆和功能分析
生物技术通报, 2018, 34(7): 101-107

ZHANG Ye-meng, SHEN Ying-fang, WANG Ying-fang, YAO Pin-ya, WANG Hai-qing
Cloning and Functional Analysis of the MtLEA5B Gene from Medicago truncatula
Biotechnology Bulletin, 2018, 34(7): 101-107

文章历史

收稿日期:2018-01-26

蒺藜苜蓿MtLEA5B的克隆和功能分析
张业猛1,2, 沈迎芳2, 王英芳1,2, 姚品雅1,2, 王海庆1     
1. 中国科学院大学西北高原生物研究所 高原适应与进化重点实验室,西宁 810002;
2. 中国科学院大学,北京100049
摘要:胚胎发育晚期丰富蛋白(Late embryogenesis abundant protein,LEA protein)是一类参与植物抗逆性应答的亲水蛋白。旨在揭示异源表达LEA蛋白在非生物胁迫条件下对宿主的保护作用。从蒺藜苜蓿(Medicago truncatula)幼苗中克隆到一个编码LEA_5蛋白的基因,命名为MtLEA5B,通过半定量RT-PCR分析MtLEA5B的表达模式。利用SDS-PAGE蛋白电泳检测重组蛋白的耐热性和可溶性。构建原核表达载体,转化大肠杆菌过表达,检测大肠杆菌的生长存活情况。MtLEA5B的表达模式受到低温(4℃)、脱水、150 mmol/L NaCl胁迫和ABA处理的调控。SDS-PAGE蛋白电泳出现一条相比理论预测值较大,且沸水条件下可溶性未降低的条带。在大肠杆菌中过表达MtLEA5B蛋白能明显提升宿主菌在高温(55℃)和冷冻(-20℃)条件下的生存率。MtLEA5B是诱导性表达基因;MtLEA5B蛋白具有较高的亲水性和耐热性。在大肠杆菌中过表达可以明显提升其对温度胁迫的耐受性。
关键词蒺藜苜蓿    胚胎发育晚期丰富蛋白    MtLEA    
Cloning and Functional Analysis of the MtLEA5B Gene from Medicago truncatula
ZHANG Ye-meng1,2, SHEN Ying-fang2, WANG Ying-fang1,2, YAO Pin-ya1,2, WANG Hai-qing1     
1. Northwest Institute of Plateau Biology, Academy of Sciences/Key laboratory of Adaptation and Evolution of Plateau Biota, xining 810008;
2. University of Chinese Academy of Sciences, Beijing 100049
Abstract: Late embryogenesis abundant protein(LEA protein)is hydrophilic one that is involved in the responses to stress in plants. This work is aimed to unravel the protective effect of heterologous expressed LEA protein on host under abiotic stress. A novel gene encoding LEA_5 protein was cloned from Medicago truncatula seedlings, and designated as MtLES5B, and its expression patterns were analyzed by semi-quantitative RT-PCR. Then the heat resistance and solubility of recombinant protein were detected by SDS-PAGE protein electrophoresis. Further, the prokaryotic expression vector was constructed and transferred to Escherichia coli so as to induce MtLEA5B over expression in E. coli. The expression patterns of MtLEA5B gene was regulated by dehydration, low temperature, high salt stress, and ABA induction. There was a band by SDS-PAGE protein electrophoresis, which was larger than the theoretical prediction value and did not decrease in solubility under boiling water condition. The overexpression of MtLEA5B protein in E. coli obviously increased the survival rate of hosts at heat(55℃)and freeze(-20℃). Moreover, MtLEA5B was an inducible expressed gene, MtLEA5B protein had high hydrophily and heat resistance, overexpression in E. coli may significantly enhance its tolerance to temperature stress.
Key words: Medicago truncatula     late embryogenesis abundant protein     MtLEA    

胚胎发育晚期丰富蛋白在种子发育后期阶段不断积累而得名[1]。LEA蛋白分布极为广泛,植物的其他组织和器官经非生物胁迫和ABA能诱导LEA蛋白的合成[2, 3],此外在蛭形轮虫、细菌、昆虫、线虫中都有发现LEA蛋白的存在[4-6]。LEA蛋白不仅能提升生命体对逆境胁迫的耐受性[7-9],而且LEA蛋白的异源表达也能提升转化子对逆境胁迫的耐受性。Wang等[10]将小麦TaLEA3导入羊草(Leymus chinensis plants)后,发现转基因羊草对干旱的耐受性明显提升。所以,LEA蛋白不仅是非生物胁迫环境因素选择的靶位点,也是局部研究植物适应机制的目标基因之一,克隆LEA明确其作用机制,对于转基因技术改良植物的抗逆性具有重要意义。

LEA蛋白根据不同的分类方法有不同的命名[11-13],Battaglia等[14]根据氨基酸同源性和保守基序将LEA蛋白分为7组。LEA_1、LEA_2、LEA_3、LEA_4、LEA_6和LEA_7组都有特定的保守基序,被视为典型LEA蛋白;而LEA_5组因为没有明显的保守基序或相似序列,被认为是非典型LEA蛋白。不同的LEA蛋白家族在功能和作用机制上并不具有明显的相似性,小麦中Em蛋白(LEA_1)体外可以防止柠檬酸合成酶的凝聚和失活[15];拟南芥中AtEm6(LEA_1)蛋白不仅是种子发育所必需,也能响应高盐胁迫[16-17]。LEA_2蛋白内在的无序结构,主要作为分子伴侣来发挥冷冻保护[18]。LEA_3可以响应多种胁迫,例如,水稻中OSLEA3(LEA_3)可以响应ABA诱导和高盐胁迫[19],玉米中的ZmLEA3(LEA_3)可以响应于高盐、低温、ABA诱导等多种途径[20-21]。LEA_5B家族的SAG21可以提升拟南芥对细菌病原体的耐受性和抗氧化能力,LEA_5A家族的Rab8可以提升玉米对干旱的耐受性[22]

目前,LEA_5蛋白作为一类非同源蛋白,其功能机制尚且十分模糊。本研究从蒺藜苜蓿A17幼苗中克隆到一个编码胚胎发育晚期丰富蛋白的基因MtLEA5B。并通过生物信息学方法对其进行分析,运用RT-PCR和原核表达对其表达模式和对宿主菌在温度胁迫条件下的保护进行验证,为转基因技术改良植物的抗逆性提供参考奠定基础。

1 材料与方法 1.1 材料

将蒺藜苜蓿A17种子用浓硫酸处理5-10 min,用双蒸水洗涤多次除去残余硫酸。处理后的种子置于含有1/2 MS固体培养基中,4℃春化2 d移至21℃、16 h/8 h光照条件下萌发。3 d后将已发芽的幼苗移栽到蛭石:营养土(3: 1)的混合基质中,在21℃、16 h/8 h光照条件下生长,每7 d用复合肥溶液浇灌1次。所用引物的合成由上海生物生工合成(上海)。

表 1 试验所用引物
1.2 方法 1.2.1 胁迫处理

将生长3周的幼苗进行非生物胁迫和ABA处理。NaCl胁迫处理:用150 mmol/L NaCl水溶液浇灌培养基质,分别于0和8 h及1和3 d取样;ABA胁迫处理:将整株幼苗取出洗净砂石后,转移至铺有多层吸水纸的培养皿中,用0.05% Tween 20(V/V)的100 μmol/L脱落酸溶液喷洒,封盖防止植物脱水,分别于0、1、3和6 h取样;脱水胁迫处理:将幼苗取出洗净砂石,温室自然脱水,分别于0、4、8和12 h取样;低温胁迫处理:将幼苗转移至4℃、16 h/8 h光照培养箱中进行培养,分别于0和8 h及1和3 d,取样。取样后迅速液氮冷冻,-80℃冰箱保存,每组处理设3次生物学重复。

1.2.2 MtLEA5B的克隆和序列分析

胁迫处理样本总RNA提取使用TRIzol®试剂(Invitrogen,USA),参照说明书使用Recombinant DNase I(RNase-free,TaKaRa,大连)去除基因组DNA。cDNA第一条链按PrimeScript ®RT reagent Kit(Perfect Real Time,TaKaRa,大连)的操作说明合成。以蒺藜苜蓿转录组cDNA为模板,MtLEA5BF/R为引物,用PyrobestTM DNA聚合酶(TaKaRa,大连)进行30次PCR扩增。使用PCR纯化试剂盒(上海生工生物,上海)纯化回收。

采用DNAMAN软件分析核酸序列;用ExPASy网站(http://web.expasy.org/translate/)分析开放阅读框OPF;用NCBI网站(http://www.ncbi.nlm.nih.gov/)进行核苷酸和蛋白质相似性检索;用SMART(http://smart.embl-heidelberg.de/)和Pfam(http://pfam.xfam.org/)进行蛋白质序列分析[23]

1.2.3 MtLEA5B原核表达载体构建

将上述PCR纯化产物经BamHⅠ和SacⅠ双酶切连接到pET30a载体中。重组质粒转化至大肠杆菌菌株(DH5α),筛选阳性克隆提取质粒,进行酶切鉴定,获得原核表达载体pET30a-MtLEA5B。

1.2.4 MtLEA5B蛋白的异源表达和热稳定性分析

将质粒pET30a和pET30a-MtLEA5B转化至大肠杆菌BL21(DE3)菌株内。在含有50 mg/L卡那霉素的LB培养皿中筛选阳性克隆,接种于10 mL含有50 mg/L卡那霉素的LB液体培养基中,37℃,180 r/min震荡培养。12 h后,按照1: 100(V: V)转接到5 mL LB培养基中培养OD600=0.6-0.8。菌液中加入异丙基硫代半乳糖苷(Isopropyl-D-thiogalactopyranoside,IPTG)至最终浓度1.0 mmol/L。诱导3 h,取1 mL pET30a和pET30a-MtLEA5B菌液作为第一组,另取1 mL pET30a和pET30a-MtLEA5B菌液作为第二组。

将2组诱导样品8 000 r/min离心2 min,弃上清,用80 mL的灭菌双蒸水重悬,第一组直接100℃金属浴10 min;第二组加入20 μL 5×SDS,100℃金属浴10 min。15 000 r/min离心5 min,取上清,第1组加入20 μL 5×SDS。各取6 μL进行SDS-PAGE进行电泳。

1.2.5 异源表达大肠杆菌转化子抗逆性试验 1.2.5.1 定性试验

将含有pET30a-MtLEA5B和pET30a载体的大肠杆菌进行液体培养和诱导,培养和诱导条件如1.2.4所述。培养诱导后的菌液OD600=0.9,用含有50 mg/L卡那霉素和1.0 mmol/L IPTG的LB液体培养基将诱导菌液分别稀释10、100和1 000倍,取原诱导菌液和稀释菌液进行胁迫处理试验。温度胁迫试验中分别用60℃金属浴温育30 min和-20℃冰箱冷冻24 h。将上述胁迫菌液和对照组菌液取10 μL滴加到含有1.0 mmol/L IPTG和50 mg/L卡那霉素的固体LB培养基中。37℃倒置培养24 h后进行观察。

1.2.5.2 定量分析

大肠杆菌转化子的培养、IPTG诱导、胁迫条件以及LB固体培养基与1.2.5.1相同,当IPTG诱导菌液培养OD600=0.9,用含有50 mg/L卡那霉素和1.0 mmol/L IPTG的LB液体培养基稀释10倍,取100 μL均匀涂于固体LB培养基中,37℃倒置培养24 h进行菌落计数,公式如下:

2 结果 2.1 MtLEA5B的克隆和序列分析

以蒺藜苜蓿幼苗样品cDNA为模板进行扩增,获得MtLEA5B的cDNA序列,长度为285 bp;编码94个氨基酸(图 1-A),预测分子量为10.05 kD,理论等电点(pI)为9.40(https://web.expasy.org/cgi-bin/protparam/protparam)。基因序列在JCVI数据库(http://www.jcvi.org/)中显示MtLEA5B是位于第1染色体编码胚胎发育晚期丰富蛋白的基因;MtLEA5B蛋白富含丙氨酸(12.72%)和缬氨酸(11.70%),而半胱氨酸和组氨酸含量为0;DNAMAN软件显示平均亲水系数为-0.472(图 1-B)。

图 1 蒺藜苜蓿MtLEA5B的序列分析 A:蒺藜苜蓿LEA5B编码区序列和氨基酸序列;B:MtLEA5B蛋白质的疏水性预测

通过进行BlastP分析,MtLEA5B蛋白包含Pfam LEA_3结构域,并且与地三叶(Trifolium subterraneum)、大豆(Glycine max)、木豆(Cajanus cajan)和菜豆(Phaseolus vulgaris)豆科植物的LEA蛋白具有较高的相似性(图 2)。

图 2 蒺藜苜蓿MtLEA5B与其他植物LEA蛋白氨基酸序列比对分析 TsLEA:Trifolium subterraneum(GAU43746.1);GmLEA:Glycine max(NP_001237596.1);CcLEA:Cajanus cajan(XP_020204737.1);PvLEA:Phaseolus vulgaris(XP_007145200.1)
2.2 MtLEA5B在非生物胁迫条件下表达特性分析

通过对不同胁迫下MtLEA5B的半定量RT-PCR分析(图 3),发现MtLEA5B表达量上调,在蒺藜苜蓿幼苗中为诱导型表达。

图 3 ABA诱导和非生物胁迫条件下MtLEA5B的RT-PCR分析 A:100 μmol/L ABA;B:150 mmol/L NaCl;C:温室脱水;D:4℃低温
2.3 pET30a-MtLEA5B载体的构建和MtLEA5B蛋白表达特性的分析

重组质粒pET30a-MtLEA5B经酶切鉴定(图 4-A),转化至大肠杆菌表达菌株BL21(DE3),进行SDS-PAGE电泳检测。结果(图 4-B)显示,MtLEA5B蛋白原核表达理论值是10.05 kD,但是发现表达重组蛋白相比于蛋白理论分子量略大,并且诱导菌液直接在无菌水中煮沸,大部分蛋白都已变性,但目的蛋白含量未发生太大变化,表明MtLEA5B蛋白的高度偏向性导致其具有较高的抗性,在SDS变性溶剂和高温煮沸条件下仍然存在部分结构。

图 4 pET30a-LEA5B表达载体酶切鉴定(A)和SDS-PAGE电泳检测(B) M:1.0 kb-plus DNA Marker和低分子量蛋白质Marker;B+S:BamHⅠ+ SacⅠ双酶切;B:BamHⅠ单酶切;S:SacⅠ单酶切;P:pET30a-LEA5B质粒;1:含pET30a-LEA5B载体的BL21经IPTG诱导后沉淀中的总蛋白;2:含pET30a-LEA5B载体的BL21经IPTG诱导后上清液中的总蛋白;3:含pET30a载体的BL21经IPTG诱导后沉淀中的总蛋白;4:含pET30a-载体的BL21经IPTG诱导后上清液中的总蛋白
2.4 MtLEA5B异源过表达对大肠杆菌的胁迫保护

为鉴定大肠杆菌中过表达MtLEA5B是否对宿主菌具有保护作用。将含有pET30a-MtLEA5B和pET30a载体的大肠杆菌BL21(DE3)分别培养至OD600=0.9,经60℃高温和-20℃冷冻胁迫处理,发现含有pET30a-MtLEA5B载体的大肠杆菌生存状态明显增多(图 5-A)。

图 5 LEA5B蛋白过表达对大肠杆菌温度胁迫的保护 A:菌液滴板试验;B:菌落形成率统计

菌落计数结果(图 5-B)显示,在60℃高温胁迫处理后,含有BLpET30a载体的菌株菌落形成率仅有4.29%,含有pET30a-MtLEA5B载体的菌株菌落形成率为13.88%;在-20℃冷冻胁迫处理后,含有pET30a载体的菌株菌落形成率仅有9.39%,含有pET30a-MtLEA5B载体的菌株菌落形成率为18.16%。

3 讨论

LEA蛋白与植物逆境胁迫耐受性密切相关,可以维持植物在极端环境条件下的正常生命代谢活动[24-27]。本研究在蒺藜苜蓿幼苗中克隆到定位于第1染色体上的MtLEA5B,编码LEA_5B类蛋白,经疏水氨基酸残基预测MtLEA5B蛋白具有较高的亲水性。MtLEA5B蛋白的氨基酸序列与地三叶、大豆、木豆等豆科植物具有高度一致性,表明LEA_5B蛋白具有种属特异性。其表达模式响应于高盐、脱水、温度胁迫和ABA处理,但MtLEA5B在不同胁迫条件下响应不尽相同,表明MtLEA5B参与蒺藜苜蓿对非生物胁迫抗性反应过程是复杂的,可能通过多种信号通路交叉进行。

虽然有实验结果对特定LEA蛋白功能和机制进行阐述,但LEA_5蛋白是一类疏水残基高于典型LEA蛋白的非同源蛋白,这些结果并不具有普适性。Baker和Galau等[28-29]发现LEA_5蛋白沸水孵化可能形成一种不溶的球形构象。本研究中,经沸水孵化后,大部分大肠杆菌细胞的内生蛋白质消失在上层清液与总蛋白提取的SDS样品缓冲中,但是MtLEA5B重组蛋白数量上并没有造成明显减少,推测原因是MtLEA5B重组蛋白质中富含的极性丙氨酸残基使具有较高热稳定性和亲水性,以及较短的氨基酸序列赋予它灵活的结构。此外,重组MtLEA5B蛋白的表观分子质量明显高于估计的分子质量,其原因可能是LEA蛋白分子的高亲水性或无序结构导致其在沸水和SDS中仍存在部分结构。

众多研究表明,异源表达LEA蛋白能够提升植物和细菌对非生物胁迫的耐受性[30-34,]。Rodriguez-Salazar等[35]研究发现LEA1蛋白响应于囊肿干燥且能提升维氏固氮菌高温耐受性。Wu等[36]发现过表达SmLEA可以增强大肠杆菌和丹参对干旱和高盐胁迫的耐受性。本研究在大肠杆菌中过表达MtLEA5B蛋白,明显增强了大肠杆菌对温度胁迫的耐受性。但是,MtLEA5B蛋白过表达在高盐和脱水条件下未发现有明显的保护作用,推测MtLEA5B对不同的胁迫有不同的响应机制[37-38]。此外有研究表明,LEA蛋白对植物的发育过程也具有调控作用[39-40]。因此MtLEA5B是否存在其他的调控机制尚且不明,需要进一步科研进行证实。

4 结论

从蒺藜苜蓿中克隆获得MtLEA5B,其受外源激素ABA和高盐、脱水和温度胁迫的调控;其编码的蛋白质属于LEA_5B蛋白家族,在SDS变性剂和高温煮沸条件下仍能保持一定的结构。MtLEA5B在大肠杆菌转化子中超表达可以增强大肠杆菌对温度胁迫的耐受性。

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